1. Field of the Invention
The present invention relates to a spectroscopic apparatus, comprising a feature of handling multiple wavelengths simultaneously and in particular relates to a configuration of a spectroscopic unit, which realizes miniaturization of an optical system and attains low polarization dependency and low insertion loss over an operating wavelength range.
2. Description of the Related Art
In the past, spectroscopic apparatuses have been used for detecting monochromatic spectra etc. mainly in measurement instruments and observation equipment. However, in recent years, because of the expansion of communication capacity by multiplexing within a wavelength range and the anticipated flexibility of system operations by using differences in wavelength, a spectroscopic apparatus based on the simultaneous utilization of multiple wavelengths has been sought.
In order to apply a device comprising a spectroscopic apparatus to an optical communication system, it is crucial to achieve low insertion loss, miniaturization and low cost, in addition to low polarization dependency, as has been required in measurement instruments in the past.
In a device, which uses multiple wavelengths simultaneously, when a spectroscopic unit (apparatus) is constituted by a diffraction grating, functional elements (such as a photodiode and a deflection switch) are disposed in an array at intervals of an desired wavelength in a dispersion direction. The interval between the functional elements must be such that they are separated in accordance with the amount of angular dispersion of the diffraction grating (the size of the dispersion angle per unit wavelength), so that the wavelength interval of the light, the spectrum of which is obtained, is separated onto the array pitch of the functional elements. This distance is one of the significant factors in determining the device size, and the large amount of angular dispersion of the diffraction grating is the key to miniaturization.
On the other hand, in the spectroscopic unit, to enhance the diffraction efficiency (the ratio of the wavelength to power concentration in the dispersive direction of the wavelength) of the diffraction grating is a key to reducing the insertion loss of the device.
However, wavelength characteristics of the diffraction efficiency of p-polarized light are different from those of s-polarized light, and Polarization Dependent Loss (PDL) occurs.
In general, it is difficult to achieve large angular dispersion and high diffraction efficiency, while maintaining low polarization dependency outside a particular wavelength range. Or when a certain wavelength range is specified, it is difficult to select any angular dispersion, which provides low polarization dependency and high diffraction efficiency. For example, in a reflective diffraction grating, an angular dispersion achieved by a grating period of 600/mm for a wavelength of approximately 1550 nm, is the angular dispersion providing low polarization dependency and high diffraction efficiency.
In view of the above problem, conventionally, the following three methods have been applied or suggested.
The first method is a common method, which has been used in the past, and is described in non-Patent Document 1. In this case, the polarization dependency is ignored, and a diffraction grating parameter such that the angular dispersion and the diffraction efficiency of an operating wavelength are prioritized for one polarization state is selected. The polarization state of the light incident on the diffraction grating is spatially dispersed by an optical material (such as rutile) for dispersing polarized light, and a part of the dispersed light is matched with the other polarization state, by using a ½ wave plate. Then, by being incident on the diffraction grating, low polarization dependency, high diffraction efficiency, and high angular dispersion can be realized.
FIG. 1 is a block diagram showing a configuration of a spectroscopic apparatus of a first conventional method.
Multi-wavelength light output from a fiber and a collimator 10 is split into p-polarized light and s-polarized light by a polarization splitter/converter unit 11, and the polarization state of the one split light is converted into that of the other by a wave plate. For example, when the spectroscopic apparatus of FIG. 1 has a configuration, which operates optimally for the p-polarized light, the s-polarized light is converted to the p-polarized light by the polarization splitter/converter unit 11. In such a manner, the width of the optical beam, which passed through the polarization splitter/converter, is expanded by a prism pair 12, and is input to a condenser lens 13. The light collected by the condenser lens 13 is reflected by an MEMS mirror array 14 and is input to the resolution lens 15. The resolution lens 15, for example, irradiates a diffraction grating 16 by the p-polarized light and the light converted from the s-polarized light to the p-polarized light. The spectra of the light are obtained by the diffraction grating 16. As is clear from FIG. 1, the diffraction grating 16 has to have a large area in order to receive the two separate optical beams. Production of the diffraction grating 16 becomes more difficult and the yield becomes lower as the area increases. Thus, if a diffraction grating with a large area were to be used, the price of the whole spectroscopic apparatus would be high. The size of the spectroscopic apparatus itself would also become large, going against the current demand for a small-sized and low-priced apparatus.
FIGS. 2A-2B and FIG. 3 are diagrams explaining a second conventional method.
The second method is a method stated in Patent Document 1 and others. The angular dispersion of a first diffraction grating is ignored, and the parameters of the diffraction grating are selected so that low polarization dependency and high diffraction efficiency can be obtained at a designated wavelength. As shown in FIGS. 2A and 2B, in order to compensate for the insufficient angular dispersion, two (or an even number) of diffraction gratings are arranged so that their angular dispersions are summed. In addition, in order to prevent PDL from occurring within a wavelength range including the designated wavelength, a wave plate is provided between the diffraction gratings. The PDL is canceled out by inverting the polarization states between the two diffraction gratings. This method allows the achievement of low polarization dependency, high diffraction efficiency, and high angular dispersion.
As shown in FIG. 2A, for the purpose that the light, the spectra of which are obtained by the diffraction grating 20, is collected by the focusing optical system 21, and is properly incident on the optical receiver element or movable reflector array 22, the spatial intervals of the light collected after spectroscopic splitting have to correspond with the array intervals of the optical receiver element or movable reflector array 22. Therefore, when the angular dispersion of the diffraction grating 20 is not sufficient, the intervals between the diffraction grating 20 and the optical receiver element or movable reflector array 22 need to be longer. However, this causes the apparatus to be increased in size. Thus, as shown in FIG. 2B, large angular dispersion is acquired by using two or more diffraction gratings 20. By so doing, the interval between the diffraction grating 20 and the optical receiver element or movable reflector 22 can be reduced, enabling the whole apparatus to be kept small. Further, in the second method, a wave plate 23 is provided between the diffraction gratings 20 to reduce the polarization dependency.
FIG. 3 is a fundamental configuration diagram of the spectroscopic apparatus described in Patent Document 1. In this configuration, instead of the two diffraction gratings 20, the light passes through the diffraction grating 20 twice, gaining the angular dispersion. The spectrum of the light entering from a port 24 is obtained by the diffraction grating 20. The spectroscopically split light passes through the ¼ wave plate 23 and is reflected by the mirror 22. The light reflected by mirror 22 passes through the ¼ wave plate 23 once again. Here, the light passes through the ¼ wave plate twice, and the polarization state of the light switches from the p-polarized light to the s-polarized light or from the s-polarized light to the p-polarized light. While in the state that the polarization has been switched, the light passes through the diffraction grating a second time. Because the light passes through the diffraction grating 20 twice, the angular dispersion is doubled; however, the polarization is switched when the light passes through the diffraction grating 20 for the first time, and again the second time. When the light passes through the same diffraction grating 20 twice in the state that the polarization is switched, the polarization characteristics of the diffraction grating 20 are canceled out. In other words, assuming that a loss that occurred for light that was p-polarized the first time is a, and a loss that occurred for light that was s-polarized the second time is b, the total loss incurred by the light passing through the diffraction grating 20 twice is a+b. On the other hand, the loss incurred by the light, which was s-polarized the first time, passing through the diffraction grating 20 twice, switching the polarization each time is b+a. Therefore, an effect that both polarization components incur the same loss after the light passes through the diffraction grating 20 twice can be obtained.
FIG. 4 is a diagram explaining the third conventional method.
The third method is described in Patent Document 2 and Patent Document 3. Like the second method, the angular dispersion of one diffraction grating is ignored, and the parameters of the diffraction grating are selected so that low polarization dependency and high diffraction efficiency can be obtained at a designated wavelength. In order to compensate for the insufficient angular dispersion, two (or an even number) of diffraction gratings are arranged so that their angular dispersions are summed. At that time, the diffraction gratings are arranged so that their grooves are perpendicular to each other. Because the grooves are perpendicular to each other, incident conditions of the p-polarized light and the s-polarized light are inverted, and the same effect as the effect obtained when a wave plate is provided between the diffraction gratings can be obtained. This method allows achievement of low polarization dependency, high diffraction efficiency, and high angular dispersion (in comparison with the second method, a merit of this method is that a wave plate is not required).
In FIG. 4, the light input from an optical fiber 25 is collimated by a collimator lens 26, and the spectrum of the light is obtained by a first diffraction grating 27. The spectroscopically split light propagates to a second diffraction grating 28. The grooves of the second diffraction grating are orthogonal to the grooves of the first diffraction grating 27. The spectrum of the light is obtained in the direction orthogonal to the first diffraction grating 27 by the second diffraction grating 28. The spectroscopically split light obtained by the second diffraction grating 28 is collected onto an array element 30 by a focusing lens 29. In such a case, the angular dispersion is not simply the sum of the angular dispersions of both the first diffraction grating 27 and the second diffraction grating 28, as the angular dispersion directions of each are orthogonal to one another. Thus, the angular dispersion becomes smaller than the simple summation.
[Patent Document 1]
U.S. Pat. No. 6,765,724
[Patent Document 2]
Japanese Patent Application Publication No. H02-61529
[Patent Document 3]
Japanese Patent Application Publication No. 2001-13006
[Non-patent Document 1]
D. M. Marom “Wavelength Selective 1xK Switching System” Optical MEMS 2003 pp. 43-44
However, the above three methods have the following problems.
In the first method, due to the polarization splitter in FIG. 1, effective areas of elements after the polarization splitter including the diffraction grating have to be twice as large, and consequently the optical elements grow in size, causing an increase in cost. Specifically, if the area of the diffraction grating doubles, a degradation of the yield generally increases by more than a factor of two, and the cost generally increases by a factor of two.
FIGS. 5A-5B are diagrams showing diffraction efficiency in the second method. The second method uses diffraction gratings with the same characteristics. Therefore, when a wavelength, at which the diffraction efficiency of the p-polarized light and that of the s-polarized light are the same, does not have a characteristic, of being symmetric on either side of the center of the operating wavelength range, wavelength dependency occurs of the entire insertion loss within the operating wavelength range. Symmetric diffraction efficiency and angular dispersion characteristics, still difficult to achieve in general, can only be realized for the operating wavelength. In other words, if a diffraction grating has the characteristics of FIG. 5A, the characteristic that the s-polarization light and the p-polarization light are switched and superimposed, is shown in FIG. 5B. In this case, the diffraction efficiency still has wavelength dependency in the operating wavelength range.
The third method has problems, in addition to the problem of the second method, such as the optical arrangement inside the device being three-dimensional (the array element is tilted at 45° to the grooves of the diffraction grating), and that the entire angular dispersion is approximately 1/√2 that of the second method, as shown in FIG. 4.